Voltage controlled oscillator

ABSTRACT

A cascaded voltage controlled oscillator is described that includes a first oscillator stage having a first oscillator stage first input, a first oscillator stage second input and a first oscillator stage output. A second oscillator stage includes a second oscillator stage input and a second oscillator stage output wherein the first oscillator stage output is input to the second oscillator stage input and wherein the second oscillator stage output is fed back to the first oscillator stage second input. A third oscillator stage includes a third oscillator stage input and a third oscillator stage output wherein the second oscillator stage output is fed to the third oscillator stage input.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to co-pending U.S. Patent ApplicationNo. ______ (Attorney Docket No. BERKP007) entitled PHASE SYNCHRONOUSMULTIPLE TANK OSCILLATOR filed concurrently herewith, which isincorporated herein by reference for all purposes.

FIELD OF THE INVENTION

[0002] The present invention relates generally to Voltage controlledoscillators (VCO's). A VCO with enhanced Q and better signal to noiseration (SNR) is disclosed.

BACKGROUND OF THE INVENTION

[0003] Voltage Controlled Oscillators (VCO's) are important componentsin wireless communication systems. FIG. 1 is a block diagramillustrating a conventional differential cascaded VCO with an LC tank.The output of stage 102 is input to stage 104 and the output of stage104 is input to stage 106. The output of stage 106 is negatively fedforward to the input of stage 102 with the positive output being fedinto the negative input and the negative output being fed into thepositive input. The cascaded stages produce a filtering effect withsomewhat improved phase noise, but the circuit tends to consume a largeamount of current.

[0004] It would be desirable if an improved cascaded VCO could bedeveloped with low phase noise so that the signal to noise ratio in theoutput signal can be reduced. Furthermore, it would be desirable if theamplitude could be increased without increasing the current drawn by thedevice.

SUMMARY OF THE INVENTION

[0005] An improved cascaded VCO is disclosed. In one embodiment, eachoscillator stage receives an additional input from a second stage inaddition to the conventional input received from a previous stage. Theinputs are selected with a phase relationship that causes the totalinput signal power to each stage to be increased.

[0006] It should be appreciated that the present invention can beimplemented in numerous ways, including as a process, an apparatus, asystem, a device, a method, or a computer readable medium such as acomputer readable storage medium or a computer network wherein programinstructions are sent over optical or electronic communication links.Several inventive embodiments of the present invention are describedbelow.

[0007] In one embodiment, a cascaded voltage controlled oscillatorincludes a first oscillator stage having a first oscillator stage firstinput, a first oscillator stage second input and a first oscillatorstage output. A second oscillator stage includes a second oscillatorstage input and a second oscillator stage output wherein the firstoscillator stage output is input to the second oscillator stage inputand wherein the second oscillator stage output is fed back to the firstoscillator stage second input. A third oscillator stage includes a thirdoscillator stage input and a third oscillator stage output wherein thesecond oscillator stage output is fed to the third oscillator stageinput.

[0008] These and other features and advantages of the present inventionwill be presented in more detail in the following detailed descriptionand the companying figures which illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

[0010]FIG. 1 is a block diagram illustrating a conventional differentialcascaded VCO with an LC tank.

[0011]FIG. 2 is a block diagram illustrating a multiple input cascadedLC tank VCO.

[0012]FIG. 3 is a block diagram illustrating a two input LC tankoscillator used in one embodiment.

[0013]FIG. 4 is a block diagram illustrating a cascaded LC tankoscillator wherein a second input is obtained for each stage by feedingforward the input from a previous stage.

[0014]FIG. 5 is a block diagram illustrating an improved oscillatorstage for use in the cascaded LC tank oscillator shown in FIG. 4.

DETAILED DESCRIPTION

[0015] A detailed description of a preferred embodiment of the inventionis provided below. While the invention is described in conjunction withthat preferred embodiment, it should be understood that the invention isnot limited to any one embodiment. On the contrary, the scope of theinvention is limited only by the appended claims and the inventionencompasses numerous alternatives, modifications and equivalents. Forthe purpose of example, numerous specific details are set forth in thefollowing description in order to provide a thorough understanding ofthe present invention. The present invention may be practiced accordingto the claims without some or all of these specific details. For thepurpose of clarity, technical material that is known in the technicalfields related to the invention has not been described in detail so thatthe present invention is not unnecessarily obscured.

[0016]FIG. 2 is a block diagram illustrating a multiple input cascadedLC tank VCO. LC tank oscillators are shown in the illustrated embodimentand are referred to extensively herein. In other embodiments, othertypes of oscillators are used and are connected in a similar manner asshown. In addition, it should be noted that each LC tank oscillator maybe single ended. In the example shown, LC tank oscillators with twoinputs are shown.

[0017] Oscillator stage 204 receives input from stage 202 in a mannersimilar to that described above. In addition, oscillator stage 204 alsoreceives as an additional input the output signal from oscillator stage206 which is positively fed back to oscillator stage 204. Similarly,oscillator stage 206 receives input fed forward from stage 204 and fedback positively from stage 208. The output of stage 208 is fed forwardnegatively to stage 202 with the positive output being input to thenegative input and the negative output being input to the positiveinput. Stage 202 also receives input from the output of stage 204, whichis fed back positively. The 45 degree phase difference between the twoinput signals to each stage of the four stage amplifier causes the inputpower to be increased and enhances the amplitude of the output. Anadvantage of this phase arrangement is that the 45 degree added signaltends grow faster than the orthogonally added noise that is introduced,which enhances the SNR.

[0018] In other embodiments, different arrangements of the input andoutput connections are used. In some embodiments, the second input isobtained from a stage that is not immediately next to the stage that isreceiving the input, i.e. stages may be skipped. In addition, differentnumbers of stages are used in other embodiments. The phase relationshipamong the inputs is arranged so that the input power is increased.

[0019]FIG. 3 is a block diagram illustrating a two input LC tankoscillator used in one embodiment. Oscillator 300 includes positiveinput nodes 302 and 304 and negative input nodes 312 and 314. The outputis taken at output nodes 322 and 324. Advantageously, NMOS transistorsare used at the input nodes to increase ramp up speed.

[0020]FIG. 4 is a block diagram illustrating a cascaded LC tankoscillator wherein a second input is obtained for each stage by feedingforward the input from a previous stage. Then inputs for stage 404 areobtained from the output of stage 402 and the fed forward input of stage402. The inputs for stages 406 and 408 are obtained in similar fashionand the inputs for stage 402 are obtained by negatively feeding forwardthe output of stage 408 and also negatively feeding forward the inputfrom stage 408. The feed forward relationship increases the ramp up rateof the signal.

[0021]FIG. 5 is a block diagram illustrating an improved oscillatorstage for use in the cascaded LC tank oscillator shown in FIG. 4.Oscillator stage 500 includes positive input nodes 502 and 504 andnegative input nodes 512 and 514. The input from the immediatelyprevious stage is applied across nodes 502 and 512 and the input fromone stage before the immediately previous stage is applied across nodes504 and 514. A combination of NMOS and PMOS devices are used. Ingeneral, PMOS devices tend be slower than NMOS. In the design shown, thePMOS devices receive the signal before the NMOS devices and can beslower. Therefore, smaller size PMOS devices (as small as the NMOSdevices) can be used. Thus, power consumption is reduced.

[0022] An improved cascaded VCO has been described. Each oscillatorstage receives an additional input from a second stage in addition tothe conventional input received from a previous stage. The inputs areselected with a phase relationship that causes the total input signalpower to each stage to be increased. In addition, improved LC tankoscillators have been disclosed for use in such cascaded VCOarchitectures.

[0023] Although the foregoing invention has been described in somedetail for purposes of clarity of understanding, it will be apparentthat certain changes and modifications may be practiced within the scopeof the appended claims. It should be noted that there are manyalternative ways of implementing both the process and apparatus of thepresent invention. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

What is claimed is: 1-6. (Cancelled)
 7. A cascaded voltage controlledoscillator comprising: a first oscillator stage having a firstoscillator stage first input, a first oscillator stage second input anda first oscillator stage output; a second oscillator stage having asecond oscillator stage input and a second oscillator stage outputwherein the first oscillator stage output is input to the secondoscillator stage input and wherein the second oscillator stage output isfed back to the first oscillator stage second input; and a thirdoscillator stage having a third oscillator stage input and a thirdoscillator stage output wherein the second oscillator stage output isfed to the third oscillator stage input; wherein the first oscillatorstages includes an LC tank oscillator.
 8. A cascaded voltage controlledoscillator as recited in claim 7, wherein the first oscillator stagefirst input and the first oscillator stage second input corresponds totwo separate signals.
 9. A cascaded voltage controlled oscillator asrecited in claim 7, wherein the second oscillator stage and the thirdoscillator stage each includes an LC tank oscillator.
 10. A cascadedvoltage controlled oscillator as recited in claim 7, wherein the firstoscillator stage includes a transistor having a source and a drain, andthe LC tank oscillator is connected between the source and the drain.11. A cascaded voltage controlled oscillator as recited in claim 7,wherein: the first oscillator stage includes a first plurality oftransistors and a second plurality of transistors; drains of each of thefirst plurality of transistors are connected; drains of each of thesecond plurality of transistors are connected; and the LC tankoscillator is connected between the drains of the first plurality oftransistors and the drains of the second plurality of transistors.
 12. Acascaded voltage controlled oscillator as recited in claim 7, whereinthe first plurality of transistors includes a first PMOS transistor anda first NMOS transistor; and the second plurality of transistorsincludes a second PMOS transistor and a second NMOS transistor.
 13. Acascaded voltage controlled oscillator comprising: a first oscillatorstage having a first oscillator stage first input, a first oscillatorstage second input and a first oscillator stage output; a secondoscillator stage having a second oscillator stage input and a secondoscillator stage output wherein the first oscillator stage output isinput to the second oscillator stage input and wherein the secondoscillator stage output is fed back to the first oscillator stage secondinput; and a third oscillator stage having a third oscillator stageinput and a third oscillator stage output wherein the second oscillatorstage output is fed to the third oscillator stage input; wherein thefirst oscillator stage first input and the first oscillator second inputare arranged to have a phase difference such that combining the firstoscillator stage first input and the first oscillator second inputresults in a combined input having a combined signal and a combinednoise, wherein the combined signal grows faster than the combined noise.